The in vitro reactivity of superoxide with a variety of substrates has been investigated. Alcohols are the major and product resulting from the reaction of alkyl halides and tosylates with an excess of potassium superoxide in dimethyl sulfoxide. The overall reaction is an extraordinarily rapid one in which the bond-forming step proceeds with inversion of configuration. A similar reaction in benzene employing 18-crown-6 ether and a 1:1 ratio of KO2/substrate produces primarily the dialkyl peroxide, ROOR, again with inversion of configuration at carbon. The reaction of carboxylic esters with potassium superoxide in benzene in the presence of 18-crown-6 ether produces, upon aqueous work-up, the corresponding carboxylic acid and alcohol in good to excellent yields by a process which appears to involve an initial nucleophilic attack of O2 ions at the carbonyl carbon and by the subsequent formation of intermediate peroxo species. The reaction of alpha-keto, alpha-hydroxy, and alpha-halo ketones, esters, and carboxylic acids with potassium superoxide in benzene in the presence of 18-crown-6 ether results in the oxidative cleavage of these compounds to carboxylic acids in a reaction which, in several respects, is reminiscent of the behavior of certain dioxygenases.